Seismic behaviour of typical unreinforced masonry (URM) brick houses, that were common in early last century in New Zealand and still common in many developing countries, is experimentally investigated at University of Canterbury, New Zealand in this research. A one halfscale model URM house is constructed and tested under earthquake ground motions on a shaking table. The model structure with aspect ratio of 1.5:1 in plan was initially tested in the longitudinal direction for several earthquakes with peak ground acceleration (PGA) up to 0.5g. Toppling of end gables (above the eaves line) and minor to moderate cracking around window and door piers was observed in this phase. The structure was then rotated 90º and tested in the transverse (short) direction for ground motions with PGA up to 0.8g. Partial out-of-plane failure of the face loaded walls in the second storey and global rocking of the model was observed in this phase. A finite element analysis and a mechanism analysis are conducted to assess the dynamic properties and lateral strength of the model house. Seismic fragility function of URM houses is developed based on the experimental results. Damping at different phases of the response is estimated using an amplitude dependent equivalent viscous damping model. Financial risk of similar URM houses is then estimated in term of expected annual loss (EAL) following a probabilistic financial risk assessment framework. Risks posed by different levels of damage and by earthquakes of different frequencies are then examined.
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Recent major earthquakes such as Northridge 1994 and Izmit Kocaeli 1999 highlighted the poor performance of existing buildings constructed prior to the early 1970’s. Low lateral seismic design coefficients and the adopted “working stress design” methodology (essentially an elastic design) lacked any inelastic design considerations, thus leading to inadequate detailing. Insufficient development lengths, lapping within potential plastic hinge regions, lack, or total absence of joint transverse reinforcement, and the use of plain round reinforcement and hooked end anchorages were common throughout the structure. The behaviour is generally dominated by brittle local failure mechanisms (e.g. joint or element shear failures) as well as possible soft-storey mechanisms at a global level. Amongst several possible retrofit interventions, a typical solution is to provide the structure with additional structural walls i.e. external buttressing or column in-fills. Extensive developments on precast, post-tensioned, dissipative systems have shown promise for the use of rocking wall systems to retrofit existing poorly detailed frame structures. In this contribution, the feasibility of such a retrofit intervention is investigated. A displacement-based retrofit procedure is developed and proposed, based on targeting pre-defined performance criteria, such as joint shear and/or column curvature deformation limits. A design example, using the proposed retrofit strategy on a prototype frame is presented. A brief overview on experimental work ongoing at the University of Canterbury investigating the dynamic response of advanced rocking walls for retrofit purposes will be provided.
Current seismic retrofit strategies generally focus on increasing the strength/stiffness or upgrading the mechanical properties of a structure or element. A typical drawback of this approach is that the demand on structural and sub-structural elements can be increased. In a previous contribution by the authors (Ireland et al., 2006) a counter-intuitive but rational seismic retrofit strategy consisting of selective weakening techniques was proposed. In this paper results of experimental investigations performed on benchmark & selectively weakened structural walls at the University of Canterbury are discussed. The experimental investigations consisted of quasi-static uni-directional tests on two benchmark and two retrofitted cantilever wall specimens. The first benchmark wall specimen was detailed as typical of pre-1970’s construction practice. An equivalent wall was retrofitted using a selective weakening approach involving a horizontal cut at foundation level to allow for a controlled rocking response. The second benchmark specimen represented a more severe scenario where the inelastic behaviour was dominated by shear. A retrofit solution involving vertically segmenting the wall to improve the ductility and retain gravity carrying capacity by inducing a flexural response was implemented. The experimental results confirmed the viability and efficiency of the proposed retrofit technique towards improving the performance of structural walls. Constructability issues and suggestions for practical implementation of the proposed retrofit solution are also discussed.
